1. Field of the Invention
The invention relates to a process devised for changing the effective speed of a polyphase asynchronous motor until it reaches a setpoint speed, the polyphase stator current of the motor having a variable frequency and its effective value being determined by a current control variable, and a device for the application of the process plus a suitable motor system for the application of the process.
2. Discussion of the Prior Art
The control of speed and torque of a polyphase asynchronous motor is per se well-known and may for example be implemented by means of an inverter.
For instance, it is known from U.S. Pat. No. 4,825,131, by Nozakietal, to compare the stator current of the motor with a current control variable in order to infer from it the stator current to be supplied by the inverter to the motor and its frequency. In this way, the speed of the motor may be brought to a desired value and be kept there, while the torque of the motor may be controlled according to the current control variable. This state of the art is a typical current limiting control for the continuous running duty of the motor whose purpose it is to provide optimum control of the motor in continuous running duty (i.e. in essentially steady conditions).
For optimum motor control in the transient running duty, i.e in conditions which change relatively fast (dynamic control of desired speed changes in the case of changing load) no provisions were made.